1. Field of the Invention
The present invention relates to an image misconvergence correction apparatus for a projection television, and more particularly to an image misconvergence correction apparatus capable of correcting a video signal misconvergence. The present invention is based on Korean Patent Application No. 2002-40430, which is incorporated herein by reference.
2. Description of the Prior Art
In general, a television set scans red (R), blue (B), and green (G) beams from one electron gun on a display device of a cathode ray tube (CRT) to render images. Therefore, in the television set, only one electron gun is controlled for the image misconvergence correction.
In the meantime, a projection television renders images through CRTs projecting images of the respective colors by the R, G, and B beams. The images projected through the respective CRTs are magnified, and the magnified images are reflected by mirror to be displayed on a large screen. Accordingly, the projection television controls the respective CRTs spaced apart from one another so that the R, G, and B beams converge at one point on the screen to implement complete color images, which is called the convergence adjustment. A video displayer such as the projection television, when the beams are not precisely deflected due to the deformations of deflection yokes controlling the respective CRTs or due to the optical locations of the respective CRTs, renders smeared or abnormal colors on the screen, which is referred to as the occurrence of misconvergence.
FIG. 1 is a view for schematically showing a display device of a general projection television. The display device has R, G, and B CRTs 1a, 1b, and 1c, a mirror 2, and a screen 3. The respective R, G, and B CRTs spaced apart from one another in a certain distance project images to the mirror 2, the projected images are reflected from the mirror 2 and then displayed on the screen 3. At this time, the images displayed on the screen 3 may have optical distortions due to the locations and angles of the respective R, G, and B CRTs 1a, 1b, and 1c. 
FIG. 2 shows shapes of the images projected on the screen 3 by the respective R, G, and B CRTs 1a, 1b, and 1c, the images being shown on the screen 3 of FIG. 1. As shown in FIG. 2, the image from the R CRT 1a is distorted to the right, the image from the B CRT 1b is distorted to the left, and the image from the G CRT 1c is distorted concave, due to the relative locations of the R, G, and B CRTs 1a, 1b, and 1c. Accordingly, the images projected on the screen 3 by the respective R, G, and B CRTs 1a, 1b, and 1c are matched to implement one image, the shape and color of the image are displayed distortedly.
FIG. 3 is a view for schematically showing a conventional convergence correction apparatus using an A-class amplifier.
The convergence correction apparatus shown in FIG. 3 has a video signal processing unit 11, a correction value generator 12, an operational amplifier 13, and a convergence yoke 21 built in a CRT 20.
The video signal processing unit 11 processes a broadcast signal externally received to output a video signal, and vertical and horizontal synchronous signals.
The correction value generator 12 is synchronized with the vertical and horizontal synchronous signals outputted from the video signal processing unit 11, and outputs a convergence correction value for a convergence correction.
The operational amplifier 13 amplifies the convergence correction value up to a high-power signal. In general, the convergence yoke coil 21a built in the convergence yoke 21 is driven by a high voltage and a high electric current to form a magnetic field, and an electron beam path is changed by the formed magnetic field. Therefore, the operational amplifier 13 may be any of A-class, B-class, and C-class amplifiers which linearly amplifies currents and voltages in most occasions. A feedback resistor 14 feeds back to the operational amplifier 13 a voltage value for current passing through the convergence coil 21a to heighten or lower an amplification degree of the operational amplifier 13.
In the meantime, the above A-, B-, or C-class amplifier is configured with a power transistor to form an amplification stage for linearly amplifying voltages and currents. The power transistor has a drawback to high turn-on resistance and power consumption due to the nature of current-driven devices.
The power efficiency of a general power transistor does not exceed 50%, and the rest of the power is converted into heat. Accordingly, the operational amplifier having the A-, B-, or C-class amplification stage has a problem of a big heat sink to be provided due to the power consumption.
FIG. 4 is a block diagram for conceptually showing a convergence correction apparatus partially compensated for the drawback to the convergence correction apparatus shown in FIG. 3.
The convergence correction apparatus shown in FIG. 4 has a video signal processing unit 32, a correction value generator 33, a D-class amplifier 34, a low-pass filter (LPF) 35, and a CRT 40 provided with a convergence yoke 41 thereon.
The video signal processing unit 32 processes an external broadcast signal and outputs a video signal, a horizontal frequency, and a vertical frequency.
The correction value generator 33 calculates a convergence correction value to correct a video signal convergence based on a convergence distortion value.
The D-class amplifier 34 inputs and amplifies the convergence correction value to a high-power signal having a predetermined voltage and current. At this time, the D-class amplifier 34 uses a field effect transistor (FET) to perform amplifications based on switching operations. The D-class amplifier using the FET has a high power efficiency compared to a method of driving a convergence yoke by the power transistor, since the D-class amplifier has very low turn-on resistance. Accordingly, the D-class amplifier 34 has low power consumption and heat generation compared to a linear power transistor. The output of the D-class amplifier 34 is filtered in a low-pass filter 35, and applied to the convergence yoke 41 built in the CRT 40. At this time, current applied to the convergence yoke 40 is converted into a predetermined voltage value by a resistor 36 and fed back to the correction value generator 33.
FIG. 5 is a view for explaining an operation concept of the D-class amplifier 34 shown in FIG. 4.
As shown in FIG. 5, the D-class amplifier 34 operates in response to a positive pulse and a negative pulse formed by inverting the positive pulse. In here, the positive pulse is a convergence correction value outputted from the correction value generator 33, and the negative pulse is obtained by inverting the convergence correction value inputted in the D-class amplifier 34.
FIG. 6 is a view for showing a waveform of an output voltage Vout outputted from the D-class amplifier 34 shown in FIG. 5.
As shown in FIG. 6, the D-class amplifier 34 is operated by two enhancement-type NMOS's 34a and 34b which are alternately turned on and off. Accordingly, noise due to voltage changes occurring at the time the respective enhancement-type NMOS's 34a and 34b are turned on and off, that is, switching noise appears in the output voltage Vout. Therefore, the switching noise is fed back to the correction value generator 33, causing a problem that the correction value generator 33 is malfunctioned by the switching noise.